Commercially available MRI devices typically consist of main magnets which are positioned in parallel and horizontally. Surface coils (also known as surface plates) are essentially loops of conducting material, such as copper tubing. Surface coils are commonly positioned in a plane being substantially perpendicular to the plane of the main magnets. The surface coils are placed directly on or over the region of interest for increased magnetic sensitivity, more information is available at http://www.mr-tip.com.
The positioning of the coil is an important determinant of performance. As only the region close to the surface coil will contribute to the signal, there is an improvement in the signal to noise ratio for these regions, compared to the use of receiver coils that surround the appropriate part of the body. These coils are specifically designed for localized body regions, and provide improved signal to noise ratios by limiting the spatial extent of the excitation or reception.
The effective free space within an MRI device, especially lab-scale and experimental MRI devices is very much limited. The distance between the scanned object and the main magnet is significantly wider as compared with the distance to the surface coils. This is truer when surface coils are utilized. Scanned objects, as such as neonates and laboratory animals, e.g., rats and mice, are preferably scanned when they are immobilized in a perfect horizontal configuration. When main magnets are positioned horizontally, surface coils are usually positioned vertically. The irregular body of the animal to be scanned is somewhat compressible along the vertical plane whilst it is substantially non compressible along the horizontal plane. Hence, either the MRI device is designed to be big enough to accommodate the object, and thus the device is much more expensive, or the scanning resolution is relatively low.
A few patents disclose means for overcoming those drawbacks, and present methods for positioning and securing the RF surface coil at a predetermined configuration. Hence for example, U.S. Pat. No. 5,085,219 ('219) discloses an adjustable holder for a magnetic resonance imaging RF surface coil for imaging a part of the body such as the temporomandibular joint. Elements are provided for positioning and securing the RF surface coil at a first predetermined point along a longitudinal axis of the holder and for positioning and securing the RF surface coil at a second predetermined point transverse to the longitudinal axis and at a radial distance R from the longitudinal axis. Patent '219 shows that the surface coil may be adjusted along a path transverse to the longitudinal axis by way of an adjustable shaft or arm which can be moved inwardly and outwardly, and the surface coil can be pivoted in a step-wise manner using a pivot head having a plurality of equally spaced detents. This technology is not implementable in small laboratory-scale MRI devices and scanned object must be tilted and otherwise maneuvered such that optimal imaging is obtained.
The magnetic field produced by the MRI device is sensitive to manufacturing process variability of the magnets as well as to the ambient temperature of the examination area. Therefore, the frequency of the magnetic field, generated by an MRD's main magnets, changes from one venue to another and from one operation to another, and even once in every few scanning operations. Radiofrequency transmitted by the RF coil assembly needs to match the main magnetic field in order to receive a good signal, low noise and sharper images. The variability in the magnetic field is compensated by tuning the frequency of the electromagnetic radiation transmitted by the RF coils. Currently, this tuning mechanism involves manually mechanically adjusting the location of the RF coils with respect to the magnetic field, thereby causing a change in the RE field. Such manual systems involve trial and error and are prone to elaborate and lengthy calibration.
None of the above provides a simple solution for precise positioning of the animal within an MRI device. Hence an MRI-compatible and positioning system fulfill a long felt need. In addition, an automated RE coil tuning unit, provided to replace manual calibration of the RE field, also fulfills a long felt need.